The discovery of superconductivity in the La-Ba-Cu-oxide system (J. G. Bednorz and K. A. Muller, Z. Physik B-Condensed Matter, Vol. 64, pp. 189-193, 1986) precipitated worldwide activity that very soon resulted in the discovery of other classes of oxide superconductors. See, for instance, M. K. Wu et al., Phys. Rev. Letters, Vol. 58(9), pp. 908-910 (1987); R. J. Cava et al., Phys. Rev. Letters, Vol. 58(16), pp. 1676-1679 (1987); D. W. Murphy et al., Phys. Rev. Letters, Vol. 58(16), pp. 1888-1890, 1987; Z. Z. Sheng et al., Nature, Vol. 332, pp. 55-58 1988; H. Maeda et al., Japanese Journal of Applied Physics, Vol. 27(2), pp. L209-L210, 1988; and U.S. patent applications Ser. Nos. 160,799 and 155,330, the former now U.S. Pat. No. 4,880,771, the latter now abandoned all incorporated herein by reference.
Although many of the newly discovered oxide superconductors have transition temperature (T.sub.c) above liquid nitrogen temperture (77 K.) and thus offer promise for widespread technological use, it was soon realized that significant problems have to be overcome before these novel materials can find substantial commercial application. In particular, it was found that bulk samples of the materials typically have a relatively low critical current density (J.sub.c). For instance, conventional bulk samples of YBa.sub.2 Cu.sub.3 O.sub.7 typically have a J.sub.c of the order of 10.sup.3 A/cm.sup.2 at 77 K. and in zero applied magnetic field, and still substantially lower in an applied magnetic field. Such a J.sub.c is generally considered to be too low for most applications.
There are at least two problems that contribute to the observed low values of J.sub.c in conventional bulk samples of high T.sub.c (by "high T.sub.c " we mean generally T.sub.c .gtoreq.30 K., preferably&gt;77 K.; by "T.sub.c " we means the highest temperature at which the D.C. resistance is zero to within experimental limits) oxide superconductors. (It will be appreciated that bulk samples consist of many superconductor grains or crystallites that are packed to form a relatively dense body). One of the two problems is the so-called "weak link" problem. This pertains to the relatively low value of current that can flow without resistance from one superconductor grain to an adjoining one. This current will be referred to as the "inter-grain" current. The other is the so-called "flux flow" problem. This pertains to the relatively low current that can flow essentially without resistance within a given superconductor grain, due to weak flux pinning. The relevant critical current density will be referred to as the "intra-grain" critical current density and will be designated herein as J'.sub.c. It will be appreciated that low values of critical current density are not an inherent property of high T.sub.c oxide superconductors since, for instance, current densities of the order of 10.sup.6 A/cm.sup.2 have been observed in thin films of YBa.sub.2 Cu.sub.3 O.sub.7.
Significant progress towards solution of the weak link problem has already been made. See S. Jin et al., Applied Phys. Letters, Vol. 52(24), pp. 2074-2076, 1988; S. Jin et al., Applied Physics Letters, Vol. 54(6), pp. 584-586, 1989; and U.S. patent application Ser. No. 126,083, now U.S. Pat. No. 5,011,823, all incorporated herein by reference. The progress resulted from the discovery of so-called "melt-textured growth" (MTG), a processing technique that comprises melting and oriented solidification of the superconductor material, resulting in highly textured material that can sustain significantly higher current densities than conventionally prepared bulk material. Bulk samples of YBa.sub.2 Cu.sub.3 O.sub.7, prepared using MTG, have exhibited critical current densities up to about 10.sup.4 A/cm.sup.2 at 77 K. in an applied field of 1 Tesla.
Recently progress has also been made in overcoming the flux-flow problem, when it was shown that irradiation of a single crystal of YBa.sub.2 Cu.sub.3 O.sub.7 with fast neutrons can raise J.sub.c of the single crystal to about 6.times.10.sup.5 A/cm.sup.2 at 77 K. and 0.9 Tesla (R. B. van Dover et al., Nature, Vol. 342, pp. 55-57, Nov. 5, 1989). However, neutron irradiation of bulk samples of superconductor would be a relatively costly and inconvenient technique for achieving high J'.sub.c in commercial applications. Thus, the need still exists for finding a method that can be used to produce material with improved intra-grain critical current density J'.sub.c, i.e., that can result in material with improved flux pinning. This application discloses such a method.
As is well known, the high T.sub.c oxide superconductors that are of relevance to this application are so-called "type II" superconductors. In type II superconductors, for magnetic fields (H) between H.sub.c1 and H.sub.c2, magnetic flux lines partially penetrate the superconductor but do not destroy superconductivity (H.sub.c1 and H.sub.c2 are the lower and upper critical fields, respectively). It is desirable that the flux lines are strongly "pinned" in the material for H.sub.c1 &lt;H&lt;H.sub.c2, since flux line movement results in energy dissipation and electrical resistance. Thus, as is well known, it is necessary to provide strong flux pinning in the material if high J'.sub.c is desired.
In conventional (low T.sub.c type II superconductors) it is known that microscopically fine defects can serve as effective pinning sites, especially if the size scale of the defects is on the order of the superconducting coherence length of the material. However, no technique (other than, possibly, irradiation with fast neutrons) for introducing effective pinning sites into high T.sub.c (oxide) superconductors is known to the art.
As is known to those skilled in the art, intra-grain critical current density in type II superconductors can be estimated by means of the so-called "Bean model" (see C. P. Bean, Reviews of Modern Physics, Vol. 36, p. 31, 1964). The model expresses the intra-grain critical current density J'.sub.c as follows: EQU J'.sub.c =30.times..DELTA.M/d,
where J'.sub.c is in A/cm.sup.2, .DELTA.M is the measured magnetization difference (in emu/cm.sup.3) between increasing and decreasing applied magnetic field in a magnetization hysteresis measurement, and d is the average grain size in cm. The grain size is typically determined by standard metallographic techniques using optical or scanning electron microscopy.